Airfoils of gas turbines, turbine rotor blades and stator vanes, require extensive cooling in order to keep the metal temperature below a certain allowable level and prevent damage due to overheating. Typically such airfoils are designed with hollow spaces and a plurality of passages and cavities for cooling fluid to flow through. The cooling fluid is typically air bled from the compressor having a higher pressure and lower temperature compared to the gas traveling through the turbine. The higher pressure forces the air through the cavities and passages as it transports the heat away from the airfoil walls. The cooling construction further comprises film cooling holes leading from the hollow spaces within the airfoil to the external surfaces of the leading and trailing edge as well as to the suction and pressure sidewalls.
In the state of the art the film cooling holes extending from cooling passages within the airfoil to the leading edge are positioned at a large angle to the leading edge surface and designed with a small length to diameter ratio. Typically, the angle between the cooling hole axis and the leading edge surface is greater than 20° and the ratio of the cooling hole length to the cooling hole diameter is about 10, typically less than 15. Such holes are drilled by a electro-discharge machining process and more recently by a laser drilling process. While such film cooling holes provide a good convective cooling of the leading edge of the airfoil due to the cumulative convective cooling area of all the film cooling holes together that are positioned between the root and the tip of the airfoil leading edge. The cooling air that exits the film cooling holes provides further cooling by means of a film that passes along the surface of the airfoil leading edge.
The establishment of a cooling film by means of a number of exit holes along the leading edge is sensitive to the pressure difference across the exit holes. While a small pressure difference can result in an ingestion of hot gas into the film cooling hole, a large pressure difference can result in the cooling air to blow out of the hole and will not re-attach to the surface of the airfoil.
Furthermore, the short length to diameter ratio of the film cooling holes and the large angle between the hole axes and the leading edge surface can lead to the formation of vortices about the exit holes. This results in a high penetration of the cooling film away from the surface of the airfoil and in a decrease of the film cooling effectiveness about the leading edge of the airfoil.
One way to provide better film cooling of the airfoil surface is to orient the film cooling holes at a shallower angle with respect to the leading edge surface. This would decrease the tendency of vortex formation. However, a more shallow angle results in a larger length to diameter ratio of the film cooling hole, which exceeds the capabilities of today's laser drilling machines.
European Patent EP 0 924 384 discloses an airfoil with a cooling construction of the leading edge of an airfoil that provides improved film cooling of the surface. The disclosed airfoil comprises a trench that extends along the leading edge and from the root to the tip of the airfoil. The apertures of the film cooling holes are positioned within this trench in a continuous straight row. The cooling air bleeds to both sides of these apertures and provides a uniform cooling film downstream and to both sides of the airfoil.